1. Field of the Invention
The present invention relates to the control of the metering process in an injection molding machine.
2. Description of the Related Art
In the control of the metering process in an injection molding machine, the screw of the injection molding machine is caused to rotate at a set rotational speed, and the screw is retracted to a set metering completion position while the molten resin pressure is controlled to a set back pressure, so that a back pressure is applied to the resin, and the resin is melted and kneaded. Then, when the screw reaches the set metering completion position, the rotation of the screw is stopped, and the metering process is completed.
Such control of the metering process includes various types of universally known control methods, such as control methods in which the rotational speed of the screw is controlled, or control methods in which both the retraction speed of the screw and the rotational speed of the screw are controlled, so that the retracting motion of the screw and the rotation of the screw stop at the set metering completion position (for example, see Japanese Patent Application Laid-Open No. S49-37955, Japanese Patent Application Laid-Open No. S55-57909, Japanese Patent Application Laid-Open No. S61-10425 and Japanese Patent Application Laid-Open No. S62-119020).
Furthermore, an invention in which pressure control is performed up to a switching position that is set slightly in advance of the completion of metering in the metering process, the control is switched to speed control from this switching position on, and the retraction speed of the screw is controlled on the basis of a set speed pattern, is also known (for example, see Japanese Patent Application Laid-Open No. H6-155534).
In the abovementioned conventional methods in which metering is performed with the screw retraction speed or screw rotational speed varied in stages, the rotation of the screw and retraction of the screw can be stopped in the metering completion position. However, immediately prior to the stopping of the retraction and rotation of the screw, the screw merely retracts and rotates in a specified pattern; accordingly, the resin pressure cannot be controlled to a specified pressure, so that variation occurs in the resin pressure. As a result, there is a variation in the quality of the molded articles.